The specificity of provirus DNA integration in Moloney murine leukemia virus induced rat thymic lymphomas was investigated. Two common regions for provirus integration in the rat genome (the Mlvi-1 and Mlvi02 locus) were demonstrated. The cellular sequences in the vicinity of the integrated provirus are conserved. Their chromosomal localization in the mouse genome was determined. Sequences homologous to Mlvi-1 and Mlvi-2 appears rearranged in a variety of tumor systems.